Support `remove_columns`

This pull request supports `remove_columns` introduced in rails/rails#8267 and should address issue #373.

`def remove_columns(table_name, *column_names)` is defined in ActiveRecord::ConnectionAdapters::SchemaStatements, which does not
require Oracle enhanced specific implementation.

`def remove_column(table_name, column_name, type = nil, options = {})`
ignores `type` and `options` arguments to avoid ORA-00904 errors.

This commit should not backport into release14 branch.